In the field of mobile communication systems, the need for higher data transmission speeds is increasing. To address such a need, work on communication standards achieving higher communication speeds than High Speed Packet Access (HSPA) has been proceeding. One example of communication standards capable of achieving such high-speed data communications is the Long Term Evolution (LTE) on which the Third Generation Partnership Project (3GPP) is working for standardization.
In LTE, some of the functions conventionally incorporated in a radio network controller (RNC) are incorporated in a base station apparatus (Evolved Universal Terrestrial Radio Access Network (E-UTRAN) NodeB (eNB)) thereby greatly enhancing the functionality of the base station apparatus. For example, when executing a handover, there can occur cases where the downlink signal buffered in the source base station apparatus (hereinafter called the eNB1) to which the mobile station apparatus as a user terminal is radio linked before the execution of the handover has not yet been transmitted to the mobile station apparatus. In such cases, the eNB1 transfers the yet-to-be-transmitted downlink signal to the target base station apparatus (hereinafter called the eNB2) to which the mobile station apparatus is to be linked after the execution of the handover. The handover scheme in which the downlink signal is transferred between the eNBs provides that the time taken to transfer the data between the eNBs be sufficiently shorter than the time taken from the moment that the eNB1 issues a handover instruction to the mobile station apparatus to the moment that synchronization is established between the eNB2 and the mobile station apparatus. The reason is that, if the time taken to transfer the data between the eNBs is sufficiently shorter, it is thought that the downlink signal transmitted to the mobile station apparatus will not be lost in part or in its entirety or that no transmission delay will occur to the downlink signal.
On the other hand, handover procedures have been proposed that are based on the LTE handover procedure and that aim to reduce data losses and delays (for example, refer to Japanese Laid-open Patent Publication No. 2008-103865). In one example of such handover procedures, the LTE handover procedure is supplemented by a handover procedure to be used when higher quality is needed, and in the case of communications demanding a high QoS (Quality of Service) class, the high quality handover procedure is used. In this handover procedure, data transfer between the eNBs is not performed, but data is transmitted from an upper node apparatus to the source and target eNBs that serve the mobile station apparatus before and after the handover, respectively.